Electrical phase shift system



Nov. 7, 1950 H. E. TOMPKINS 2,529,117

ELECTRICAL PHASE SHIFT SYSTEM Filed Aug. 30, 1945 a Sheets- Sheet 1 [N V EN TOR. HOWARD E. TOMPK/NS A 7' TORNEKS Nov. 7, 1950 H. E. TOMPKINS 2,529,117

ELECTRICAL PHASE SHIFT SYSTEM 0 O O N r 3 g 0 l v l [N VEN TOR. HOU/J/Pflf-TONP/(l/YS BY 0W 0,41, 7

Nov. 7, 1950 v H. E. TOMPKINS 2,529,117

ELECTRICAL PHASE SHIFT SYSTEM Filed Aug. 30, 1945 6 Sheets-Sheet 3 I INVENTOR. H WARD E. TOMPK/MS Nov. 7, 1950 H. E. TOMPKINS ELECTRICAL PHASE SHIFT SYSTEM 6 Sheets-Sheet 4 Filed Aug. 30, 1945 S m 3 MT 5% M n v .m m m M M WWW Nov. 7, 1950 H. E. TOMPKINS 2,529,117

ELECTRICAL PHASE SHIFT SYSTEM Filed Aug. 30, 1945 I 6 Sheets-Sheet 5 INVENTOR. mum a. TOIIPXWS BYOMMILWM Nov. 7, 1950 H. E. TOMPKINS 2,529,117

ELECTRICAL PHASE SHIFT SYSTEM Filed Aug. 30. 1945 6 Sheets-Sheet 6 0 0 o 8 52 S G W T v i 1 30K 50K I00 K a *4 z m v INVENTOR. HOA/ARO E T MPK/N Patented Nov. 7, 1950 ELECTRICAL PHASE ,SHIFT SYSTEM Howard E. Tompkins, Media, 'Pa., assignor, by mesne assignments, to Philco Corporation,- Philadelphia, Pa., a corporation of Pennsyl- Vania Application August 30, 1945, Serial No. 613,457

This'invention relates to an electrical phase shifting system. More specifically, it relates to a network or system of networks for providing two signals which differ in phase from one another by a specified difference in phase-angle over a wide range of frequencies. In certain types of electronic equipment, such as equipment used for communication purposes or recording purposes and for measurement purposes, it at times becomes desirable to obtain two signals of thesame frequency and amplitude and differing in phase-angle'from one another by some constant prescribed or specified angle over a range of frequencies. For example, such a sys tem is'desired in the helical-method of recording speech and other intelligence upon a round magnetic wire, as hereinafter described. Likewise, such a system is required in certain automatic power follower devices, such as those described in-copending application Serial No. 603,027, filed iluly -l8;'1945, now Patent No. 2,469,750, May 10, 1949.

" Previous to my invention it has been possible to provide '90 of phase-.shift'between two signals atone frequency, but it'lias not been convenient to provide such a phase-shift over a wide percentage range of frequencies. My invention enables' the production of an arbitrary phase-shift over a wide band of frequencies instead of at one frequency only. In order to accomplish this purpose, two channels are employed. One of these channels is designed in such a manner that it has a phase-shift curve with respect to frequencies which .is continually increasing or continually decreasing in phase as the frequency increases. This first network, furthermore, has the characteristic that it does .not have variations in the attenuation of the input signal as a function of frequency, That is, for .a given input signal amplitude, the same output signal amplitude is obtained at all frequencies Within the useful band. Y

=. The second channel has a similar characteristicin regard .to phase, except that over a, prede- :terminecl range of frequencies the phase-angle differs from the phase-angleof the first channel by the prescribed amount. Consequently, if a common signal is fed into the two channels, the ,output signals from the two channels will be of equal amplitude but will differinvphase a prescribed phase-angle over the range of frequencies involved.

In such a system suitable phase-shifting networks for the two channels must be designedwhich over a range of frequencies diff r in their phase-shift characteristics by a prescribed angle.

Ihave discovered that in order to design such a pair of networks, it is exceedingly helpful if a network is first designed which will have a linear phase characteristic when the phase-angle 7 Claims. (Cl. 118- 44) v2 is plotted against the-logarithm .of-the frequency. Another Way of stating this characteristic is to say that the phase-angle is proportional to the logarithm of the frequency. For convenience this characteristic may be termed a logarithmic phase characteristic. If a network is available with such a "logarithmic phase characteristic, then it is always possible to design another net work rwhich has anyexactly similar phase-characteristic except that the phase characteristic of this new networkis-sh-ifted in frequency ethat is, the plot of the-phase characteristic on ;-:the new, .or-second, netW-orkmay be copied from the phase characteristic of the first network by translating the curve -for the firstnetworkrin the direction :of the logarithmic frequency.

. Once the first network is available, the design of the second network is accomplished by well known andsim-ple network theory. Thus, to design the second network from the first network, the inductance of the first network is multiplied by-a suitable constant and the ca-pacitances are divided by the same suitable constant; the constantwused'being determined from the amount of shift along the logarithmic frequency axis that it is desired to obtain. From the above, itwill be observed that according 'to my invention, the problem of providing two channels with {la-99 or other specified phase-angle difference between the two channels is easily solved when the problem of the network with a logarithmic phase characteristic has been solved.

This latter problem I have solved with a lattice-type structure. In one form of my lattice structure I use a-pair of resistors for the series pair of arms and a pair of capacitors for the shunt pair of arms. In such a structure the phase-angle of the lattice can be shown to be proportional to the'arctangent of the ratio of the'actualfrequency to the frequency at which the value of the resistance-is equal to the value of the -reactance of the capacitor-in the lattice. When the phase-angle of sucha network is plotted against the logarithm of frequency, it can be shown that this curve contains a point of inflection. It is well known that when several networks are properly connected in tandem, the phaseshift of the overall network can *be obtained from the phase-shift of the individual networks by the simple process of adding the phase-angles of the individual networks. In order to obtain a logarithmic phase characteristic, I use several networks of'the'basic'type whichI have'discussed above, choosing the frequency of the points of inflection to'lce different in the successive networks, so that when'l .add the phase-angles of all of the networks togethen'l obtain a resultant logarithmic Phase characteristic over a considerab e p rtion of the. frequency spectrum? Of In joining together the individual lattice structures, it becomes necessary to make sure that the proper conditions are met at the junctions of the lattice structures. Thi I have accomplished in several ways. By the useyof a differential-type amplifier, I can assure the proper matching of one lattice with the next. Likewise with two lattices I can insure proper matching by arranging carrying out the helical method of recording,

the audio signal is fed into both of the two channels of my invention. The signal which comes from the output of channel one is caused to' record upon the magnetic wire on one transverse axis, and the signal whichcomes from channel two is caused to record upon the transverse axis of the magnetic wire which is at right angles to the first axis. The two channels are arranged to have a phase difference of substantially 90 over the audiofrequency range, or over that part of the frequency range which it is desired to record. Thus the recording is accomplished upon the wire in such a fashion that the magnetization of the wire progresses spirally along the wire, never going to zero in all of its directions while a signal is present.

When a signal has been recorded on a round ma netic'wire by the helical method, as herein described, it may be reproduced from the wire by the use of a system involving a single transverse magnetic pickup device, regardless of the relative orientation of the wire and the pickup.

In reproducing signals recorded in the conventional non-helical transverse manner by the helical method, two pickup devices are arranged to pick up the applicable portions of the transversely recorded signals at right angles to each other from the magnetic wire. Signals from these'two devices are passed through two separate'channels. These channels are so desi ned that they have a difference in phase shift of 90 over the recorded signal frequency ran e. Thus the si nal which come from the out ut of these two channels are substanti lly in time phase with one another, or are 180 out of phase with each other de ending u on which direction is considered. and the am litude of the resulting output is independent of the o ientation of the the desired audio-amplifier syst m.

Thus, an object of my invention is to provide a network having a phase-shift characteristic curve which is linear with respect to the logarithm of frequency over a band of frequencies.

Another object of my invention is to provide a novel system for obtaining a prescribed phase shift between twosignals over a wide frequency range.

Another obiect of my invention is to provide means for recording in a transverse direction on a round magnetic wire 'in accordance with the wire. A selected out ut thus obtained is fed into helical method of recording, thus providing a recordingwhich may be reproduced by a single transver e pickup regardless of the orien a i n of the 'wire.

Another object is to provide novel electronic unit means for reproducing a magnetic signal transversely recorded on a wire by a device which is insensitive to the orientation of the wire.

Other objects and purposes of my invention will become apparent upon a study of the drawing in which Figure 1 represents .a basic lattice phase-shifting network.

Figure 2 represents another of the possible forms of phase-shifting network.

Figure 3 .gives the phase-shift curve for the device of Figure 1 plotted against the logarithm of the frequency. H

Figure 4 shows two such phase-shift curves, displaced from each other, along the logarithmic frequency axis.

Figure 5 is one type of two-channe1,two-lattice structure for carrying out my invention, the input signal of which is derived from a magnetic signal transversely recorded on a wire.

Figure 6 shows the phase-shift curve forthe circuit of Figure 5. v

, Figure '7 is another form of two-channel, twolattice structure. V

' Figure 8 shows still another form of a twochannel, two-lattice structure.

Figure 9 shows a two-channel, three-lattice structure, as applied to helical recording.

Figure 10 shows the phase-shift curve for Figure 9.

Figure 11 shows a four-lattice, single-channel structure using cathode followers for couplings.

The basic lattice structure which I shall use in describing my invention is shown in Figure 1. This lattice is composed of two resistors, l and 2, and two capacitors, 3 and 4. There are a pair of input terminals 5, and a pair of output terminals 6. If a voltage Em is applied to the input terminal 5, the output voltage Eout which appears at the output terminal 6 can be readily computed by circuit theory. When this calculation is carried out the following equation is obtained:

v (1 +jw0R This equation can readily be simplified by cancelling out the common factor in the numera- Euut=Ein (1) tor and denominator. This is Equation 2:

1-jwCR EW -E... (2)

Since in'this equation R and C appear only as work is 90. Thus it is possible to define we as When Equation 3 is substituted in Equation 2, Equation 4 results:

Since this equation is given in terms of a ratio of odt in (5) By' a simple transformation of complex-number theory, Equation 5 can be re-writterr-in the form of Equation 6.

On first consideration Equation 6 appears to be more complicated than Equation 5, but nevertheless Equation 6 illustrates the fact that there is no change in amplitude between the input and the output signal-because the exponent of .6 is imaginary. This imaginary exponent indicates a phase shift only has occurred with no change in amplitude between the input and output signals. Further inspection of Equation 6 makes it evident that the phase shift of the network can be written as in Equation 7,. as the arc tangent of the normalized frequency, that is, the arc tangent of the frequency divided by the frequency at which there is 90 phase shift.

This phase-shift function is plotted in Figure 3", using a logarithmic scale for the ratio of the frequency to the'critical frequency.

Other networks have similar phase-shift characteristics. For example, the network-of Figure 2, which is comprised of two resistors I and 8, andtwo inductors Band III, will have an identical phase shift characteristic, assuming resistanceless inductors.

There are likewise other networks which-have phase-shift characteristics which are useful for the purposes to be'described here. However, in view of the complexity which might be introduced by-citing examples of many of these, I have limited myself in the description to follow to two networks built upon the network of Figure 1. The curve of Figure 3 has the characteristics which are desirable for the'purpose of myinvention; that is, the phase curve is'continually decreasing and over-a certain region issubstantially linear as a function of the logarithm of the frequency; It is to be noticed in this curve of Figure-3 that the point of inflection H of the curve occursat the 90 point; that is, when the frequency is equal to the critical frequency. This point of inflection H, then, comes in the middle of the substantially linear part of thecurve. The logarithmic plot of Figure 3 has the advantage that in transforming this curve to a curve where the actual frequency is plotted instead of the ratio of frequenc to the inflection frequency, it is only necessary to locate the inflection frequency on the new scale'of logarithm of frequency and then place the curve of Figure 3 on the new coordinate paper in such a manner that the inflection frequency on the new curve sheet is at the same place as the point II on the curve of Figure 3.

The curve of Figure 4 shows how a resultant curve might look after this has been done. Figure 4 shows two curves in which two inflection frequencies are used, fci and fez. In obtaining the two phase-shift curves I2 and I3, a logarithmic frequency scale is laid out in the same manner and to thesame scale as the logarithmic frequency ratio scale in Figure 3. Likewise, a phase-angle scale is laid' out on the same scale used in Figure 3 for the phase-angle scale. Then the curve-of Figure 3 is placed over the coordinate axes of Figure 4 and point on Figure 3 is located so that it coincides with-point I4 on Figure 4; Then curve I2 is drawn which is now thephase-shift curve for a network of the type of Figure l wherethe inflection frequency is given by the frequency fci.

In order to determine the shape andlocation of curve I3, this procedure-is repeated placing the point of inflection I I over the point I5 on curve'4. Then curve I3 is drawn as the curve corresponding to the curve of Figure 3.

Inspection of Figure 4 will showthat'foraregion between 45 and 135, there is substantially a constant phase shift between the two curves; thus for two networks of the character of Figure 1, and with the same voltage impressed on the two networks at the input terminals, the'voltages at the output terminals from the two networks will have substantially a constant phase difference with respect to each other over a region of frequencies lying approximately between for and fcz'f Furthermore it is to be observed that the greater the tolerance in the possible variation of phase angle with frequency, the wider the frequency band over which this network combination will give me the desired result.

For most purposes, however, the curves of'Fig' ure 4 do not give a sufiiciently wide range of frequency over which the phase angle will be substantially constant. Consequently it becomes necessary to cascade two or more networks of the type of Figure 1.

A system which employs two cascaded bridge networks is shown in Figure 5. In this figure, bridges I6 and I'I are cascaded with each other; They are coupled one to another with the aid of vacuum tubes 2%]. This combination forms one channel having a single phase-shift curve, as is shown by curve 22, in Figures. The other channel in this system is comprised of bridges I8 and I9 coupled together by vacuum tube amplifiers "2|. This channel has a single phase-shift curve as is given by curve 23 in Figure 6.

In order to more clearly demonstrate'the phase difference between the outputs ofthese 'two channels, this phase difference is plotted'in Figure'fi as curve'24. It" is to be ob'served'that phase separation between the two circuits'changes at'the lower and upper ranges of frequency. However; over a substantial range of frequencies between about cyclesp'er second and about'7000'cycles per second the angular difference between the curves stayswithin 7 of the phase separation for which this, system was designed. This isa much better constancy of phase than has been obtained previously with such simple apparatus.

Several items of interest and importance remain to be analyzed in connection with Figure- 5. The input signal to bridge I5 is obtained'froni pick-upcoil 25 whichreceives' a signal from a transversely recordedmagneticsignal on wire 2-6. Likewise, the input signal to bridge 'I 8 is obtained from pick-up coil 2'! oriented perpendicularly with respect to piCk-up-Eiwhich reproduces another component ofthe signal recorded on wire 26.; One end of each of these two pick-up coils is 'shunted to ground by capacitor 28, and likewise'is supplied with a positive bias potential for th'e operation of the arnplifie-rsflfl andEI. This positive potential is necessary in: the particular: ampli fier circuit shown, which is-aparticular type of differential amplifier. In other circuitsrnegative or no bias might be required.

Representative values of resistorsand capacitors arem'arked on bridges: I'B a'n'd I8. "These are the values which have been chosen to" ive-the result of Figuret'and they 'co'nstitutaa specific example of the invention. These values have been chosen so that-the point of inflection on the phase curvefor-bridge-lfi occurs at 1500 cycles per netic flux from moving wire 26 is supplied directly to the input of 'the bridges without the interposition of any substantial series impedance .The output from bridge I6 is applied to the amplifier tubes 20. These tubes operate as a differential amplifier in accordance with well known principles. Since this amplifier circuit does not amplify the average voltage to ground of the input terminalsbut Only amplifies the voltage difference between the two input terminals at the control grids of these two tubes 20, it does not matter that the input signal through these amplifier tubes is decidedly unbalanced with respect to ground. Similarly, amplifier tubes 2i operate in conjunction with bridge circuit l 8 as a differential amplifier. The output of tubes is fed into the bridge circuit I! and similarly the output of tubes 2| is fed into the bridge circuit l9. These bridge circuits operate in somewhat the same way as bridge circuits I6 and I8, but there are two substantial differences.

First of all, the critical frequencies, or frequencies of inflection in the phase curve of bridge circuits I1 and I9, are chosen at '75 cyclesper second and 400 cycles per second respectively. Thus the two bridge circuits I6 and I? are cascaded, and the two bridge circuits l8 and l9 are cascaded so as to give the phase curve of Figure 6.

Bridge circuits l1 and I9 supply low impedance loads. This means that these bridge circuits are substantially short-circuited. Furthermore, these circuits are fed from vacuum tubes which have a very high internal impedance. Thus the bridge circuits are effectively fed by current sources.

' It can be shown by electrical theory that the characteristic of such a bridge circuit when fed by a current source and loaded with a very low impedance is substantially the same as the characteristic of this same bridge circuit when fed by a voltage source and working into an open circuit. Thus the phase formula of Equation 7 and the phase curve of Figure 3 are still applicable to this bridge.

One of the output terminals of each of the bridges l1 and I9 is grounded. In the case of bridge 11, this ground is through capacitor 28, and in the case of bridge l9 through capacitor 29-. impedance is low compared to the tube output impedance.

The output signal from bridge I! is applied to one terminal of another differential amplifier including tubes 30; and the output of bridge circuit I 9 is applied to the other grid input terminal of the amplifier formed by tube 30. The differential amplifier formed by tube 30 subtracts the two signals from these two circuits.

As applied to simple transverse recording, the following is the action of this circuit as shown in Figure 5: The voltage picked up by coil is substantially 90 out of mechanical phase but otherwise in time phase with the voltage picked up by coil 21, for every one of the frequency com- This makes use of the fact that the bridge ponents constituting the intelligence recorded upon wire 26. I w

If the recording on the wire 26 is so oriented that it is perpendicular to the effective axis of coil 25, none of the signal is picked up by that coil,'but coil 21 is fully effective. If the recording is rotated 90, it is ineffective to generate signals in coil 21, while coil 25 is fully effective. The problem which is solved by this invention arises from the fact that when the wire is rotated to intermediate positions, the simple vectorial addition of the two signals would produce distortion. In fact, the amount of signal picked up by one coilis proportional to the cosine of the angle which the signal record makes with the coil axis, while the amount of signal picked up by the other coil is proportional to the sine of the same angle; that is, the angle between the recording and the axis of the first coil.

Thus, if the recording were oriented at 45 with respect to-both coils, the amplitude of the vectorial sum of the two signals, which are, remember, in phase, would be 141% of its value when the wire is oriented in the plane of either one of the coils. v

The present invention solves the problem thus presented by passing the outputs of the two coils 25 and 2! through a network to produce two output signals, the one originating at one pickup differing in phase by 90 with respect to the other. The two signals are then added vectorially, and the result is a true reproduction of the amplitude of the signal recorded on the wire 26. This result occurs in practice, and may be proved mathematically, but a simple example will make it immediately evident. If all the signal is picked up by one coil, whether or not it is delayed 90, the amplitude-will be correct. If the orientation of the recording is 45 with respect to both coils, so-that the relative amplitudes of the two signals picked up are the same but toolarge for direct in-phase vectorial addition, if the signal in one channel be converted from a sine function of time to. the corresponding cosine function, it may be added vectorially to the original sine function of time in the other channel to produce an output of correct amplitude. I Graphically, this constant amplitude is the radius of the circle around which the moving vector travels as time proceeds. Because a 90 time delay of the whole signal does not produce an audible effect, it simplifies the system to use the output in this form, but of course the whole signal supplied by my system may be restored to its original form by passing it through further phase delay networks of any convenient type, if required. a

.In the system shown in Figure 5, each of these frequency components picked up by coil 25 is fed through the upper channel as shown, and is applied'to grid 3| of differential amplifier 30. Similarly the components picked up by coil 2! are fed through togrid 32 of differential amplifier 30.

However, the phase shift from coil 25 to grid 3| is different than the phase shift from coil 2'! to grid 32; and this 90 phase difference in transmission path is held substantially constant over the audio spectrum. Consequently, even thoughfor every frequency picked up by coils 25 and 21, the voltages picked up are 90 out of phase with respect to each other, the voltages delivered to grids 3| and 32 are added vectorially in the differential amplifier 30. It is this latter condition which is desired, because then the voltage appearing betweengridsfil -and is the system is proportional to the signal'recordedon Wire-26.

Figure '7 shows another device illustrating my method of v accomplishing this purpose.

same pick-up coils 25 and 21 are used to pick up signals from the wire 26, and the same output.

diiferential amplifier built around tubes 30 is used. The bridge networks, however, are slightly different. In the upper channel which feeds from coil 25 to grid 3 I, two bridges 33 and 34 are used- Bridge 33 is arranged to have thesame critical frequency as bridge, I6; and bridge 34 is arranged to have the same critical frequency as bridge I1.

.However, the, impedance level, that is, the impedance of all of the elements at any particular frequency is one-tenth as great in bridge 33 as it is in bridge l6. Nevertheless, the impedance looking into bridge 33 is still sufficiently high compared to the impedance of coil 25, so that bridgecircuit 33 is substantially fed by a voltage source. Furthermore, the impedance elements in bridge 34 have been chosen to be higher in impedance than those of bridge ll, so that the impedance looking from bridge 33 into bridge 34 is quite high compared to the impedance looking from brid e 34 to bridge 33. Thus bridge 33 acts substantially as if supplying an open circuit; and bridge 34 is substantially fed by a constant voltage source. H

The output of bridge 34 is applied to thegrid 3| and consequently is substantially working into an ,open circuit. One terminal of the output of The;

- an amplified-signal; The amplifier discriminates.

bridge 34' is grounded by capacitor 35. This means that all of the system, including coil 25, bridge 33 and bridge 34, is unbalanced with respect to ground. This should cause no difficulty in the audio frequency range with the sizes of components used in this example, provided customary engineering skill is employed in the physical construction of this unit.

Similarly, the channel leading from pick-up coil 26 to grid 32 is constructed of two' bridges 36 and 31. These have the same critical frequencies as bridges i8 and I9, but have other modifications in the same way that bridges 33 and 34 are modified in comparison with bridges I6 and 11. Again the output of the lower channel is applied m nd 32 of the output differential amplifier 30, and the output from the system becomes the vec torial' sum of the voltages picked up by coils 25 and 21 from wire 26.

The circuit of Figure 7 is operative only if the impedances to ground from pickup coils 25 and 21 are high compared to the impedances of bridges 34 and 31. However, if this is not possible, for example, if one side of these coils must be grounded, the circuit of Figure 8 can be used. Here the bridges in the upper channel are the same-as the bridges-in upper channel in Figure 7, namely, bridges 33 and 34. Similarly, the bridges inlower channel are the same as the bridges in Figure 7, namely,v 36 and 31. However, the outputs of bridges 34 and .31 are treated differently than they are in Figure 7.

In Figure 8 the output of bridge 34 is unbal-x 10 only the; difference in voltage between th two gridsappliedto it appears acrossthe output and: between either output terminal and ground as,

against the; average potential to ground.

- The plate terminal 40 of one of the tubesis. used as the output terminal for thisdifferential amplifier, and the output is fed from this. intogrid- 31- of the finaldifferential amplifier 311i.v A. similar arrangement is used in the lower channel, feedingfrom coil 21 through bridges 36 andv 3,7 to diiferential amplifier 41, thefoIutput. from the plate 42 of this difierential amplifier feeding into the grid 32. This circuit. then enables one" side of each, of the pick-up; coils to be grounded and stillallows. the use of, the bridge system of phase adjustment.

In place. of the final differential amplifier tube 30, voltage adding or'resistance mixing, such as. is. commonly employed in audio program mixers; may be used. That is, the'output. signals from! tubes 39 and. 41 may be added or subtracted to achieve the desired result, inasmuch as they are. 90 out of phase throughout. the usefulrange of; frequencies, v

The diiferential amplifier is illustrated in allv these circuits because it is aparticularly convenient way to achieve the result, as it provides high input impedance, gain, and mixing, alLin one. unit..

Under certain circumstances, the, phase-shift. curve obtained by Figure 6 will not cover asufiicient frequency range. Then it becomes -desi'r.-' able to use three bridges in tandem for each channel. An arrangement, of, this character is shown in Figure 9, as applied to therecoilding processior helical recording. Here inthe upper. channel, two bridges 43 and 45 are placed in tandem, just as bridges 33 and 34' are placed in tandem in Figure 8. Bridge 43 is a low impedance bridge and. bridge 45 is a high impedance bridge in accordance with the principles set forth in connection with bridges 33 and 34 in Figure 8.

The output of bridge ,45, is fed into a difler ential amplifier 41 but this differential amplifier is now loaded on both plates with bridge 49., This bridge 49 is fed from a. high impedance cir= cuit and feeds into a low impedance circuit following the principles setdown for a bridge of the type of bridge H in connection with Figure 5. Thus, the bridge 49 feeds into the recording coil 5| which records on th wire 53. Y

The impedance of coil 5| shouldat'. all. fre-. quencies in the audio spectrum for this application be low compared to the impedance looking back into the output of bridge 49, in order to meet the requirements for this bridge, since it'is fed from a current source-the diiferential amplifier 41. I v

The lower' channel is composed of a similar chain of velementstwobridges 44 and 46, followed by a differential amplifier 48, followed'by another bridge 50, and feeding into recording coil 52. In this circumstance, bothchannelsare fed from a common source which, in order topro vide a low impedance source, is a cathode fo1- lower circuit 54 which is itself driven by the audio signal to be recorded. l

The purpose of the device of Figure 9- is to produce a helical recording which may be reproduced by an ordinary single transverse magnetic pickup regardless of the orientation or rotari motion of the wire recording. The term helical" has been chosen as a reminder of the nature of the signal recorded on the wire. As will be unv11 derstood upon consideration ofthe description of helical reproduction, a"single sine wave tone of 'It- 'is' ap arent'tnst ma ywa at-ions constant amplitude frequencywill ,be re-= corded bythis process as a constant amplitude transverse magnetization of the wire, itsorienta tion rotating at such'a r'atejthat the number of complete rotations persecond passing the pick upfcoil is the same 'as the. number of cycles per second of the tone-xvariations of amplitude of the tone are recorded as variations in intensity of the magnetization, while variations in frequency cause a change in pitch offthe helix. 'When complex notes are recorde d-, the above described effectsare superimposed.

Figure'lo is the curve for the system of Figure 9." The upper channel has a phase shift in accordancewith curve 5 in Figure 10. The lower channel has a-phase'shift in accordance with curve SB in Figure' .10. Consequently, the phaseangle 'difference between these two curves is given by curve 51. Inspection for this CurVe indicates that from a frequency of about 45 cycles per second to 12,000 cycles per second, thephase angle does not vary more than from the normal value. of 90, and that for most of the range the phase angleis considerablycloser to 90 than thistolerance of 10. I

"Such a tolerance is sufficiently good for helical recording; but for other purposes it may be desirable toobtain better frequency-phase characteristics. 7 These I may be Obtained by slight changesiirr the selection of the critical frequencies, or by an extension of the" number of bridge circuits whichareused.

, The'combination of va pair offbrid ges'feeding one intoan'other, such as. bridges 33 and 34 in Figure 8 or Figure '7, can be extended considerably with the aid'of cathode followers. Thus, in Fig ure 11 two brid'gesifland 59,- being a low and a,

high impedance bridge respectively, feed, into a pairof cathodefollowers 60 and BI. These cath ode followers are simply impedance changingdevices,. 'that .is, they changethe impedance from a, high impedance looking back into the output terminals ofbridge 5 9 into-a lowimpe'dance look-f ing into the output terminals of the two cathode followers Thus bridge 62 starts 'the chain over again asfar. as impedance is, cor1 cerned; that is, bridge. .62 'i's'f-ed bya low impedance source. This inturn feeds bridge 'B3.,'which in'turn, in this cir-' cuit, feeds the differential amplifie output system 6 4, which isused in order to correct the unbalancetoground. 1 v f circuit arrangement ,could continue on for a considerable number of bridge combina: tions.."{ However, in practice probably four tandem bridges will be. the most that will be required for'the strictest requirements.

In the design of tandem bridges for use in t is type of network, the fundamental problem is the proper choice ,of critical frequencies for'bridges be placed in tandem.

-When two 'bridge'networks are, in each side of a double-channel systemas'in Figure; 5; fq 'examme, I have determined graphically that the ratio of the frequency'bf'bridge 16 to the frequency of bridge -11 should-be about ;1 to 5 7 that is the criticalfrequency of bridg 16 should be one (1 compared to the critical frequencyof bridge l8, which Similarly, the ratio 'of' the critical frequency of bridge 16 to that of bridge I' T should be about 13 to 1 for a very smooth characteristic, and about 15 to l'for a slightly less smooth phase characteristic but a slightly broader band.

tolerances imposed' by suchusage. T I have described my invention ofa' ph ase' shifting system'withrespect to a specific set'fbfi applications, namely, that used in helical recording and reproducing. However, m invention should not be limitedto' this applicationf nor should it be limited to the particular 'enib'odim'ents which I haveshown inxthese drawings-T 1 should like my invention to be described bythe" following claims. r c

Iclaim: 1. A dual network comprising. two separate signal channels, eachof said'channels having a pair of input terminals for '"impressing' signalsf thereon anc'a pair'of" output terminals, "a pai'rj of terminals of one of said channels being connected to the corresponding pair of terminals of the other channel, said networks'being in 'lattice form and consisting of distinct branches; some of said branches consisting solely of pure resistors and others of saidbranch'es consist ing solely of substantially pure reactors to pro? vide phase shifts 'dilfering from each otherby ninety degrees over a predetermined frequency range without substantial attenuation, eachchan-f nel comprising a series of lattices in cascade, the general relationbetween the phase-anthesi'gnjal each latticev and the logarithm'offrequency of the signal 'b'eing substantially linearovera wide frequency'band each lattice,the values of the resistors and reactors being lected to provide inflection points of therelat'ion between the phase and the logarithm of f'refquency whichare different in thedifferent chan nels, and amplifying tubes which couple said lattices in each cascadechain. v

2. A dual network .comprising* two "s'eparate signal channels, each of said channels having a pair of input terminals forimpressing signals thereon and a pair of output terminals, a 'pair of'terminals of one'of said channelsbeingfcori nected t'o the corresponding pair of terminals oil the other channel,.said networks beingin lattice form' and consisting of distinct branchessgrne of saidbranches consisting solely of pure resistors and others of said branches consisting 5 1e,.

ly of substantially pure reactors to provide'phase shifts differing from each other by ninetyf'degrees over a predetermined frequency range with-- out substantial attenuation, each channel-comprising a series of lattices in cascade, the gen;

er'al relation between'the phase of the-signal in each lattice and the logarithm of frequency of the signal being substantially linear overa'wide frequency band in "each lattice; the values o'f the resistors and reactors being 'sel'ected to provideinflection points of the relation between" 'the= phase and the logarithm of frequency whichare said channels and each having output terminals, a pair of terminals of one of said channels being connected to the corresponding pair of terminals of the other channel, each of said channels comprising an all-pass network, each of said channels comprising a plurality of networks connected in tandem, each network consisting solely of resistors and reactors and each of the networks in a channel shifting the phase angles of the input signals by predetermined amounts, the infiection points of the relation between the phase and logarithm of frequency in each network in a channel being different for effecting frequency shifts over a relatively wide frequency range, the values of the reactors of the networks of the second of said channels differing from the values of the reactors of the first channel to shift the phases of the signals by amounts which differ from the shifts effected by said first channel by a substantially fixed amount over said predetermined frequency range, and a coupling amplifier connecting each of the networks in a channel to the adjacent network.

l. In an electrical circuit for generating a plurality of signals having a fixed phase angle dific-rence from each other over a predetermined range of frequencies comprising two channels, each having an input circuit for impressing signals in said predetermined frequency range on said channels and each having output terminals, a pair of terminals of one of said channels being connected to the corresponding pair of terminals of the other channel, each of said channels compri an all-pass network, each of said chancomprising a plurality of networks connected in tandem, each network consisting solely of resistors and reactors and each of the networks in a channel shifting the phase angles of the input signals by predetermined amounts, the infiection points of the relation between the phase and logarithm of frequency in each network in a channel being different for effecting frequency shifts over a relatively wide frequency range, the values of the reactors of the networks of the second of said channels differing from the values of the reactors of the first channel to shift the phases of the signals by amounts which differ from the shifts effected by said first'channel by a substantially fixed amount over said predetermined frequency range, said reactors being capacitors, and the general relation between the phase of the signals and the logarithm of the frequency being substantially linear over a wide frequency band, and a coupling amplifier connecting each of the networks in a channel to the adjacent network. 7

5. In an electrical circuit for generating a plurality of signals having a fixed phase angledifference from each other over a predetermined range of frequencies comprising two channels, each having an input circuit for impressing signals in said predetermined frequency range on said channels and each having output terminals, a pair of terminals of one of said channels being connected to the corresponding pair of terminals of the other channel, each of said channels comprising an all-pass network, each of said channels comprising a plurality of networks connected in tandem, each network consisting solely of resistors and reactors and each of the net works in a channel shifting the phase angles of the input signals by predetermined amounts, thev inflection points of the relation between the phase and logarithm of frequency in each network in a channel being different for effecting frequency 14 shifts over a relatively wide frequency range, the values of the reactors of the networks of the second of said channels differing from the values of the reactors of the first channel to shift the phases of the signals by amounts which differ from the shifts effected by said first channel by a substantially fixed amount over said predetermined frequency range, said reactors being inductors, and a coupling amplifier connecting each of the networks in a channel to the adjacent networks 6. In an electrical circuit for generating a plurality of signals having a fixed hase angle difference from each other over a predetermined range of frequencies comprising two channels, each having an input circuit for impressing signals in said predetermined frequency range on said channels and each having output terminals, a pair of terminals of one of said channels being connected to the corresponding pair of terminals of the other channel, each of said channels comprising an all-pass network, each of said channels comprising a plurality of networks connected in tandem, each network consisting solel of resistors and reactors and each of the networks in a channel shifting the phase angles of the input signals by predetermined amounts, the inflection points of the relation between the phase and logarithm of frequency in each network in a channel being different for effecting frequency shifts over a relatively wide frequency range, the values of the reactors of the networks of the second of said channels differing from the values of the reactors of the first channel to shift the phases of the signals by amounts which differ from the shifts effected by said first channel by a substantially fixed amount over said predetermined frequency range, the phases of the signals being shifted by amounts which differ by substantially ninety degrees, and a coupling amplifier connecting each of, the networks in a channel to the adjacent network.

7. In a signalling system for effecting shifts in phase angles of signals over a predetermined frequency range comprising a plurality of lattices connected in series and having an input circuit for receiving signals of a predetermined frequency range, each of said lattices in said series circuit being composed solely of resistors and reactors, and each having a linear relation of the phase shifts of the signals to the logarithm of the frequency, the inflection points of the relation between the phase and logarithm of frequency being different from that of the other lattices in said series circuit for producing a phase shift over a relatively wide frequency range, said lattices having a substantially zero attenuation for the signals over said predetermined frequency range, said reactors being inductors, and an electron tube coupling adjacent lattices.

HOWARD E. TOMPKINKS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Num er Name Date 2,174,166 Plebanski Sept. 26, 1939 2,229,450 Garman Jan. 21, 1941 2,392,476 Hodgson Jan. 8, 1946 FOREIGN PATENTS Number Country Date 439,977 Great Britain Dec. 18, 1935 

